In general, a liquid developer for electrophotography is prepared by dispersing an organic or inorganic pigment or dye such as carbon black, nigrosine or phthalocyanine blue and a natural or synthetic resin such as an alkyd resin, acrylic resin, rosine or synthetic rubber in a liquid having a high electric insulating property and a low dielectric constant, such as petroleum aliphatic hydrocarbon, and further adding a polarity-controlling agent such as metal soap, lecithin, linseed oil, higher fatty acid or vinyl pyrrolidone-containing polymer to the resulting dispersion. In such a developer, the resin is dispersed in the form of insoluble latex grains having a grain size (diameter) of from several nm to several hundred nm. In a conventional liquid developer, however, the soluble dispersion-stabilizing resin and the polarity-controlling agent are insufficiently bonded to the insoluble latex grains, so that the soluble dispersion-stabilizing resin and the polarity-controlling agent are freely dispersed in the liquid developer with ease. Accordingly, the soluble dispersion-stabilizing resin would be split off from the insoluble latex grains after storage of the liquid developer for a long period of time or after repeated use thereof, so that the grains would thereafter defectively precipitate, coagulate or accumulate, or the polarity would thereby become indistinct. Since the grains once coagulated and accumulated are difficult to re-disperse, the grains would remain to be adhered to everywhere in the developing machine, and, as a result, cause stain of images formed and accident of the developing machine such as clogging of the liquid-feeding pump.
In order to overcome such defect, a means of chemically bonding the soluble dispersion-stabilizing resin and the insoluble latex trains has been disclosed in U.S. Pat. No. 3,990,980. However, the liquid developer disclosed was still insufficient, although the dispersion stability to spontaneous precipitation of the grains could be improved in some degree. When the liquid developer was actually used in a developing apparatus, the toner adhered to the parts of the apparatus solidified to form a film thereon, and the thus solidified toner grains could hardly be redispersed. In addition, the solidified toner grains caused stain of the images duplicated and troubles in the apparatus. Accordingly, the liquid dipersion as disclosed in U.S. Pat. No. 3,990,980 was found to have a defect that the re-dispersion stability was still insufficient for practical use.
In accordance with the method of preparing the resin grains as disclosed in U.S. Pat. No. 3,990,980, there is an extreme limitation on the combination of the dispersing stabilizer to be used and the monomers to be insolubilized, in order to prepare monodispersed grains having a narrow grain size distribution. Mostly, the resin grains prepared by the method would contain a large amount of coarse grains having a broad grain size distribution, or would be polydispersed grains having two or more different mean grain sizes. In accordance with the method, it is difficult to obtain monodispersed grains having a narrow grain size distribution and having a desired mean grain size, and the method often results in large grains having a grain size of 1 .mu.m or more, or extremely fine grains having a grain size of 0.1 .mu.m or less. In addition, the dispersion stabilizer to be used in the method has another problem in that it must be prepared by an extremely complicated process requiring a long reaction time.
In order to overcome the aforesaid defects, a method of forming insoluble dispersion resin grains of a copolymer from a monomer to be insolubilized and a monomer containing a long chain alkyl moiety, so as to improve the dispersibility, re-dispersibility and storage stability of the grains, has been disclosed in JP-A-60-179751 and JP-A-62-151868 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, a method of printing a large number of prints of 5000 or more prints has recently been developed, using an offset printing master plate by electrophotography. In particular, because of further improvement of the master plate, it has become possible to print 10,000 or more prints of large size of electrophotography. In addition, noticeable progress has been made in shortening the operation time in an electrophotomechanical system, and the step of development-fixation in the system has been conveniently accelerated.
Under such situation, the dispersion resin grains as prepared by the method disclosed in the aforesaid JP-A-60-179751 and JP-A-61-151868 were found still unsatisfactory with respect to the dispersibility and the re-dispersibility when they were applied to rapid development. In addition, when they were applied to a process wherein the fixation time is shortened, or to a process using a master plate of large size (for example A-3 size or more), they were found also unsatisfactory with respect to the printing durability.
Accordingly, a need has existed to overcome the problems of the aforesaid conventional liquid developers.